Virtual memory allows processes to be larger than physical memory by storing portions of processes that don't fit in RAM on disk. When a process attempts to access memory not currently in RAM, a page fault occurs, swapping the needed page in from disk while another process runs. Hardware and software mechanisms like page tables, TLBs, and replacement algorithms efficiently manage mapping virtual addresses to physical locations and swapping pages between disk and RAM. This improves system utilization by allowing many processes to reside partially in memory simultaneously.

1. Chapter 8
Virtual Memory
Operating Systems:
Internals and Design Principles, 6/E
William Stallings
Patricia Roy
Manatee Community College, Venice, FL
©2008, Prentice Hall

2. Hardware and Control
Structures
• Memory references are dynamically
translated into physical addresses at run
time
– A process may be swapped in and out of main
memory such that it occupies different regions

3. Hardware and Control
Structures
• A process may be broken up into pieces,
which do not need to be located
contiguously in main memory
• It is not necessary for all pieces of a
process to be loaded in main memory
during execution of the process

4. Execution of a Program
• Operating system brings into main
memory a few pieces of the program
• Resident set - portion of process that is in
main memory
• An interrupt is generated when an address
is needed that is not in main memory
• Operating system places the process in a
blocking state

5. Execution of a Program
• Piece of process that contains the logical
address is brought into main memory
– Operating system issues a disk I/O Read
request
– Another process is dispatched to run while the
disk I/O takes place
– An interrupt is issued when disk I/O complete
which causes the operating system to place
the affected process in the Ready state

6. Improved System Utilization
• More processes may be maintained in
main memory
– Only load in some of the pieces of each
process
– With so many processes in main memory, it is
very likely a process will be in the Ready state
at any particular time
• A process may be larger than all of main
memory

7. Types of Memory
• Real memory
– Main memory
• Virtual memory
– Memory on disk
– Allows for effective multiprogramming and
relieves the user of tight constraints of main
memory

8. Thrashing
• Swapping out a piece of a process just
before that piece is needed
• The processor spends most of its time
swapping pieces rather than executing
user instructions

9. Principle of Locality
• Program and data references within a
process tend to cluster
• Only a few pieces of a process will be
needed over a short period of time
• Possible to make intelligent guesses about
which pieces will be needed in the future
• This suggests that virtual memory may
work efficiently

10. Support Needed for Virtual
Memory
• Hardware must support paging and
segmentation
• Operating system must be able to do the
management the movement of pages
and/or segments between secondary
memory and main memory

11. Paging
• Each process has its own page table
• Each page table entry contains the frame
number of the corresponding page in main
memory
• A bit is needed to indicate whether the
page is in main memory or not

12. Paging

13. Modify Bit in Page Table
• Modify bit is needed to indicate if the page
has been altered since it was last loaded
into main memory
• If no change has been made, the page
does not have to be written to the disk
when it needs to be replaced

14. Address Translation

15. Two-Level Hierarchical Page
Table

16. Page Tables
• Page tables are also stored in virtual
memory
• When a process is running, part of its
page table is in main memory

17. Address Translation

18. Inverted Page Table
• Used on PowerPC, UltraSPARC, and IA-
64 architecture
• Page number portion of a virtual address
is mapped into a hash value
• Hash value points to inverted page table
• Fixed proportion of real memory is
required for the tables regardless of the
number of processes

19. Inverted Page Table
• Page number
• Process identifier
• Control bits
• Chain pointer

20. Inverted Page Table

21. Translation Lookaside Buffer
• Each virtual memory reference can cause
two physical memory accesses
– One to fetch the page table
– One to fetch the data
• To overcome this problem a high-speed
cache is set up for page table entries
– Called a Translation Lookaside Buffer (TLB)

22. Translation Lookaside Buffer
• Contains page table entries that have
been most recently used

23. Translation Lookaside Buffer
• Given a virtual address, processor
examines the TLB
• If page table entry is present (TLB hit), the
frame number is retrieved and the real
address is formed
• If page table entry is not found in the TLB
(TLB miss), the page number is used to
index the process page table

24. Translation Lookaside Buffer
• First checks if page is already in main
memory
– If not in main memory a page fault is issued
• The TLB is updated to include the new
page entry

25. Translation Lookaside Buffer

26. Translation Lookaside Buffer

27. Translation Lookaside Buffer

28. Translation Lookaside Buffer

29. Page Size
• Smaller page size, less amount of internal
fragmentation
• Smaller page size, more pages required
per process
• More pages per process means larger
page tables
• Larger page tables means large portion of
page tables in virtual memory

30. Page Size
• Secondary memory is designed to
efficiently transfer large blocks of data so
a large page size is better

31. Page Size
• Small page size, large number of pages
will be found in main memory
• As time goes on during execution, the
pages in memory will all contain portions
of the process near recent references.
Page faults low.
• Increased page size causes pages to
contain locations further from any recent
reference. Page faults rise.

32. Page Size

33. Example Page Size

34. Segmentation
• May be unequal, dynamic size
• Simplifies handling of growing data
structures
• Allows programs to be altered and
recompiled independently
• Lends itself to sharing data among
processes
• Lends itself to protection

35. Segment Tables
• Starting address corresponding segment
in main memory
• Each entry contains the length of the
segment
• A bit is needed to determine if segment is
already in main memory
• Another bit is needed to determine if the
segment has been modified since it was
loaded in main memory

36. Segment Table Entries

37. Segmentation

38. Combined Paging and
Segmentation
• Paging is transparent to the programmer
• Segmentation is visible to the programmer
• Each segment is broken into fixed-size
pages

39. Combined Paging and
Segmentation

40. Address Translation

41. Protection Relationships

42. Fetch Policy
• Determines when a page should be
brought into memory
• Demand paging only brings pages into
main memory when a reference is made to
a location on the page
– Many page faults when process first started
• Prepaging brings in more pages than
needed
– More efficient to bring in pages that reside
contiguously on the disk

43. Placement Policy
• Determines where in real memory a
process piece is to reside
• Important in a segmentation system
• Paging or combined paging with
segmentation hardware performs address
translation

44. Replacement Policy
• Which page is replaced?
• Page removed should be the page least
likely to be referenced in the near future
• Most policies predict the future behavior
on the basis of past behavior

45. Replacement Policy
• Frame Locking
– If frame is locked, it may not be replaced
– Kernel of the operating system
– Key control structures
– I/O buffers
– Associate a lock bit with each frame

46. Basic Replacement Algorithms
• Optimal policy
– Selects for replacement that page for which
the time to the next reference is the longest
– Impossible to have perfect knowledge of
future events

47. Basic Replacement Algorithms
• Least Recently Used (LRU)
– Replaces the page that has not been
referenced for the longest time
– By the principle of locality, this should be the
page least likely to be referenced in the near
future
– Each page could be tagged with the time of
last reference. This would require a great
deal of overhead.

48. Basic Replacement Algorithms
• First-in, first-out (FIFO)
– Treats page frames allocated to a process as
a circular buffer
– Pages are removed in round-robin style
– Simplest replacement policy to implement
– Page that has been in memory the longest is
replaced
– These pages may be needed again very soon

49. Basic Replacement Algorithms
• Clock Policy
– Additional bit called a use bit
– When a page is first loaded in memory, the
use bit is set to 1
– When the page is referenced, the use bit is
set to 1
– When it is time to replace a page, the first
frame encountered with the use bit set to 0 is
replaced.
– During the search for replacement, each use
bit set to 1 is changed to 0

50. Clock Policy

51. Clock Policy

52. Clock Policy

53. Comparison

54. Behavior of Page Replacement
Algorithms

55. Basic Replacement Algorithms
• Page Buffering
– Replaced page is added to one of two lists
• Free page list if page has not been modified
• Modified page list

56. Resident Set Size
• Fixed-allocation
– Gives a process a fixed number of pages
within which to execute
– When a page fault occurs, one of the pages of
that process must be replaced
• Variable-allocation
– Number of pages allocated to a process
varies over the lifetime of the process

57. Fixed Allocation, Local Scope
• Decide ahead of time the amount of
allocation to give a process
• If allocation is too small, there will be a
high page fault rate
• If allocation is too large there will be too
few programs in main memory
– Processor idle time
– Swapping

58. Variable Allocation, Global
Scope
• Easiest to implement
• Adopted by many operating systems
• Operating system keeps list of free frames
• Free frame is added to resident set of
process when a page fault occurs
• If no free frame, replaces one from
another process

59. Variable Allocation, Local Scope
• When new process added, allocate
number of page frames based on
application type, program request, or other
criteria
• When page fault occurs, select page from
among the resident set of the process that
suffers the fault
• Reevaluate allocation from time to time

60. Cleaning Policy
• Demand cleaning
– A page is written out only when it has been
selected for replacement
• Precleaning
– Pages are written out in batches

61. Cleaning Policy
• Best approach uses page buffering
– Replaced pages are placed in two lists
• Modified and unmodified
– Pages in the modified list are periodically
written out in batches
– Pages in the unmodified list are either
reclaimed if referenced again or lost when its
frame is assigned to another page

62. Load Control
• Determines the number of processes that
will be resident in main memory
• Too few processes, many occasions when
all processes will be blocked and much
time will be spent in swapping
• Too many processes will lead to thrashing

63. Multiprogramming

64. Process Suspension
• Lowest priority process
• Faulting process
– This process does not have its working set in
main memory so it will be blocked anyway
• Last process activated
– This process is least likely to have its working
set resident

65. Process Suspension
• Process with smallest resident set
– This process requires the least future effort to
reload
• Largest process
– Obtains the most free frames
• Process with the largest remaining
execution window